$x runif(1, -1, 1) * y_i^2 * (runif(1, -1, 1) * sin(y_i) + runif(1, -1, 1) * cos(y_i))
$y runif(1, -1, 1) * x_i^2 * (runif(1, -1, 1) * sin(x_i) + runif(1, -1, 1) * cos(x_i))